This invention relates broadly to the control of thermal energy conversion power plants, and more particularly to a system for optimizing the net power and energy levels generated thereby based on altering the heat transfer process of the heat exchangers included therein.
Thermal energy conversion power plants, generally of the closed cycle variety similar to that disclosed in U.S. Pat. No. 4,104,883 issued to Fredrick E. Naef on Aug. 8, 1978, include two heat exchangers--an evaporator and a condenser. A working fluid, such as ammonia, for example, is heated to evaporation in the evaporator and then passed through a turbine which is mechanically coupled to an electrical generator. As the working fluid expands through the turbine, it causes the turbine to rotate and drive the generator to produce electrical energy at some electrical power level. Some of the power produced by the turbine-generator system is resupplied to the plant to drive circulating pumps and other auxiliarly equipment which operate the plant cycle. The remaining or net power produced by the plant is generally provided to an external load. In exiting the turbine, the working fluid is cooled in the condenser and thereafter returned, generally under control of a feedpump system to the evaporator where the cycle is repeated.
In Ocean Thermal Energy Conversion (OTEC) type power plants, warm water, usually from the surface of the ocean, is pumped by a circulating pump, for example, through heating tubes of the evaporator and then discharged. Heat from the warm water is transferred through the tube walls of the evaporator to the working fluid for the evaporation thereof. Similarly, cool water, usually taken from depths greater 1000 feet beneath the surface of the ocean and which may have a temperature differential from the surface water on the order of 35.degree.-45.degree. F., is pumped by another circulating pump, for example, through cooling tubes of the condenser and then discharged. Heat from the ammonia exiting the turbine is transferred to the cool circulating water through the tube walls of the condenser causing a condensation of the ammonia flowing through the condenser. Because of the relatively small temperature difference between the surface and subsurface ocean waters, the thermal efficiency of these OTEC plants, in general, is extremely sensitive to changes in the heat transfer operations occurring primarily in the evaporator and condenser.
Degradation of the thermal efficiency of OTEC power plants is primarily caused by a phenomenon known as biofouling which is endemic to OTEC type power plants. Biofouling results from bioactivity in the ocean water promoted by nutrients present therein. In the process of biofouling, marine growth is deposited on the surfaces of the tubes of the heat exchanger through which ocean water flows. In time, the deposited layer of growth builds up and acts to increase the thermal barrier between the ocean water and working fluid of the plant ultimately diminishing the heat transfer coefficient therebetween. Consequently, the net output power produced is reduced commensurate with the accumulation of biofouling activity in the tubes of the heat exchangers.
Generally, in conventional power plants, the pumping velocity of the cooling and heating circulating fluid of a heat exchanger is held substantially fixed by maintaining the power to the circulating pumps at a fixed level. However, when this conventional control approach is applied to OTEC type power plants, a great deal of energy in pumping the ocean water through the tube or shell, as the case may be, of the heat exchangers may be, at times, wasted because of the biofouling phenomenon. What makes this particularly significant is that there exists, mainly due to the relatively small temperature differential in the ocean water, a circulation flow in the OTEC plants of 20-50 times that of an equivalent power producing conventional plant. This requires the supply of a commensurate amount of pumping power therefor. Thus, in an OTEC plant, the power supplied to the circulating pumps becomes a major portion of the gross output power produced thereby. And, accordingly, any improvement in the efficiency of operation of these circulating pumps will ultimately result in a proportional increase in the amount of net power produced by the OTEC plant, thus improving the overall plant efficiency. In view of this understanding, it is apparently desirable to provide a system which can maintain an optimum pumping power level for the circulating pumps during the operation of the OTEC plant based on the adverse influence of the biofouling activity on the net power production thereof.
In another aspect, it is proposed that, at some point in time during the biofouling degradation of the net power output of the OTEC plant, the plant is to be shutdown for the purposes of cleaning the buildup of growth in the tubes of the heat exchangers as a part of the plant's maintenance procedures. Inasmuch as this be the case, it appears economically advantageous to determine the optimum plant operational duty cycle with respect to plant maintenance cleaning time. For example, if the plant is shutdown for cleaning too frequently, the plant operating time is expected to be much less than the cleaning time rendering a relatively low net energy output. Conversely, if the plant is shutdown for cleaning too seldom, it is expected that the net power will diminish substantially over the time the plant is operative due to the biofouling accumulation in the heat exchangers; and, based qualitatively on the anticipated growth buildup over the extended plant operating time, it is also expected that it will take longer for the cleaning process. Therefore, the seldomly cleaned OTEC plant is not expected to yield much more net electrical energy than that which is cleaned too frequently. It follows that a system which monitors the plant's activities for determining the operation and cleaning times of which yield an optimum plant net energy output is additionally desirable.